The present invention relates generally to a hierarchical control system for Molecular Beam Epitaxy (MBE).
Numbers in square brackets []refer to the Bibliography at the end of this "BackGround" section.
In MBE, the shutters and furnaces are commonly controlled by computer during growth. A recipe is input by the MBE operator. This recipe consists of a list of layers which will compose the thin-film. For each layer, the shutters that will be open and the time they will be open is specified. The operator chooses the furnace temperatures and layer growth times to obtain the specific thicknesses and compositions for each layer using calibration data manually obtained during a period of time prior to the Growth Period (Growth), called the Setup Period (Setup).
During the Setup, the operator prepares the machine and may run one or more of several experiments to calibrate the performance of the MBE machine. Calibration data includes Beam Equivalent Pressure (BEP) Curves which relate the Knudsen Cell Temperature and the flux, and Reflective High Energy Electron Diffraction (RHEED) measurements which indicate the rate of growth of different materials.
Beam Equivalent Pressure (BEP) curves are commonly measured manually. Measuring these curves takes a couple hours for each cell. As a result they are measured infrequently (sometimes only once a month). The temperature of the Knudsen Cell is stabilized at three or four different values within the operating range and the beam equivalent pressure, which corresponds to the flux, is measured using an ion gauge at each temperature.
The Proportional, Integral, Derivative (PID) Controllers are typically calibrated manually using Ziegler-Nichols Tuning Methods. A single set of constants (Proportional Band, Integral Time and Derivative Time) is determined and is used to control the temperatures over the entire operating range of the cell. Manually finding these values is time consuming and as a result the MBE system is tuned infrequently (rarely more than once per month), further reducing the performance of the MBE Machine.
The growth rate of different combinations of materials is also measured using Reflective High Energy Electron Diffraction (RHEED). Information on the stoichiometry and growth rate of different combinations of materials is obtained using this technique. These measurements are used to select the temperatures of the Knudsen Cells to obtain the desired compositions as well as the growth times for the layers in the recipe. To make the same thin-film three weeks later, most likely the furnace temperatures and growth times will have changed due to the changing characteristics of the MBE machine. For instance, as the material in a Knudsen Cell is consumed, the temperature of the Knudsen Cell must be increased to obtain the same flux.
Determining the furnace temperatures and layer growth times during Setup is a burden on the operator who sometimes makes mistakes. Also estimating the growth times to obtain a desired thickness is not optimal because the MBE machine's behavior changes, i.e. more or less time may be necessary to complete a layer.
A transient in the flux as shown in the typical flux wave in FIG. 1 occurs whenever the shutter is opened due to a reduction in the radiant heat reflected from the shutter after the shutter is opened. As a result, the composition of the thin-film is not correct at interfaces. Ideally the flux should immediately stabilize at one value when the shutter is open, as in the ideal flux wave in FIG. 1.
FIG. 2 shows the basic layout of a Knudsen Cell 20, which comprises a crucible 22 having an aperture 23 and a shutter 26. A heater 24 surrounds the lower part of the crucible 22 containing a melt 25. A thermocouple 28 senses the temperature of the melt. The amount of flux emitted from the cell is largely a function of the surface temperature of the melt. The melt temperature is controlled by a PID controller that regulates the amount of power sent to the heaters around the outside of the crucible. As shown in FIG. 3, the PID controller 34 observes the thermocouple temperature signal via line 33 and controls the power sent to the heaters so that the thermocouple signal follows the setpoint command signal via line 31. The output from the PID controller 34 is a power command on line 35 to an SCR power controller 36, which supplies a signal on line 37 to control the heater power in the Knudsen cell 20. The aperture in the Knudsen cell helps to provide a coherent flux beam, and the shutter allows the operator to block the flux beam when growth is not desired.
When the shutter opens, additional power is lost from the Knudsen Cell to the surrounding environment via the flux beam. This results in an increased temperature gradient from the back of the cell to the melt surface. The PID controller compensates for this increased thermal loading by increasing the average power sent to the cell to maintain a constant thermocouple reading at the back of the cell. However, since the PID controller typically senses only the back of the cell, it cannot detect the drop in melt surface temperature with respect to the thermocouple temperature. Thus, even though the PID controller maintains a nearly constant thermocouple reading throughout the shutter opening disturbance, the melt surface cools because of the increase in thermal power lost to the environment. This cooling results in an exponential drop in flux from the cell immediately after shutter opening as shown in FIG. 6.
Ideally, in situ sensing of beam flux will allow accurate closed-loop compensation of this cooling phenomenon. Unfortunately, no such in situ sensing scheme presently exists that does not interfere with satisfactory growth of thin-films on the substrate. For example, sensing the flux accurately may require the sensor to be placed directly in the flux beam; this would result in a significant disturbance in the flux beam on its way to the substrate. In the case of hot filament sensors such as ion gauges, the outgassing from these filaments could result in contamination of the film growth. Therefore, feedforward or open-loop compensation techniques are required for this situation.
Mechanical modifications are being explored by various MBE equipment manufacturers in order to reduce the shutter transient problem. For instance the shape of the shutter facing the aperture can be convex so that heat is not reflected directly back into the Knudsen Cell. This would reduce the difference in load between when the shutter is open and when the shutter is closed. Mechanical modifications to the Knudsen Cell will reduce flux transients however they will not remove them. Also hardware upgrade of existing machines is costly.
Software modifications have been explored as described by Chilton [2] and Vlcek [3] with good success. They have not however addressed in the literature the issues necessary to incorporate this capability into a manufacturing system. Integration with a recipe growth routine, automation of the process identification routine and data validity checking are issues that they have not addressed.
Even after the transient dies out, the flux is not completely stable. Noise on the thermocouple signal, non-optimal choice of parameters for the PID Controllers and the accuracy limit of the PID controllers cause the flux to vary. In current MBE applications, the PID Controllers are loaded with one set of P.I and D values for the entire temperature range.
Recipes for new materials are developed to a large extent by trial and error. An estimate of the recipe to obtain a desired behavior is grown. The thin-film is then removed from the MBE machine and characterized. If the behavior is not achieved, then the recipe is adjusted and another wafer is grown. This process is continued until the behavior is achieved. In well known material systems the number of cycles of this procedure is not very high (perhaps 3 or 4), but for new material systems 30 repetitions may be necessary. These calibration runs are very expensive because both the machine time and the substrate material are expensive.
The following United States patents are of interest.
U.S. Pat. No. 5,205,900--Inoue et al
U.S. Pat. No. 5,185,288--Cook et al
U.S. Pat. No. 5,169,798--Eaglesham et al
U.S. Pat. No. 5,096,533--Igarashi
The patent to Igarashi describes a molecular beam epitaxial growth device and molecular beam control method for exactly controlling thickness and composition of epitaxial film. The remaining patents are of less interest.